Polysilicon surface micromachining uses planar fabrication process steps common to the integrated circuit (IC) fabrication industry to manufacture microelectromechanical or micromechanical devices. The standard building-block process consists of depositing and photolithographically patterning alternate layers of low-stress polycrystalline silicon (also termed polysilicon) and a sacrificial material such as silicon dioxide. Vias etched through the sacrificial layers provide anchor points to a substrate and between the polysilicon layers which are patterned to build up mechanical elements of the device layer by layer by a series of deposition and patterning process steps. The silicon dioxide layers can then be removed by exposure to a selective etchant such as hydrofluoric acid (HF) which does not attack the polysilicon layers to release the completed device for movement thereof.
The result is a construction system generally consisting of a first layer of polysilicon which provides electrical interconnections and/or a voltage reference plane, and up to three additional layers of mechanical polysilicon which are used to form mechanical elements ranging from simple cantilevered beams to complex systems such as an electrostatic motor connected to a plurality of gears. Typical in-plane lateral dimensions can range from one micron to several hundred microns, while the layer thicknesses are typically about 1-2 microns. Because the entire process is based on standard IC fabrication technology, hundreds to thousands of devices can be batch-fabricated, fully assembled (without any need for piece-part assembly) on a single silicon substrate.
The present invention provides an advance in the art of silicon micromachining by overcoming impediments that have heretofore prevented the development of a MEM devices having five levels of polysilicon. The five-level polysilicon process as disclosed herein allows the formation of MEM devices of a greatly increased complexity as compared with the prior art. As an example of the utility of the five-level polysilicon process of the present invention for forming complex MEM devices, a MEM transmission is disclosed that can be used to control or interlock mechanical coupling between an electrostatic motor and a self-assembling MEM structure in the form of a hinged or pop-up mirror.
An advantage of the method of the present invention is that a residual stress buildup can be overcome which has heretofore prevented the formation of MEM devices using five layers of polysilicon.
Another advantage of the present invention is that MEM structures or devices can be formed with a level of complexity and functionality heretofore unattainable.
A further advantage of the present invention is that MEM structures or devices having a high level of complexity can be formed without a need for piece part assembly.
Yet another advantage is that a MEM apparatus (herein termed a transmission) can be formed using the five-level polysilicon process of the present invention to provide control over the transfer of power from a motive source such as a MEM motor to a load such as a self-assembling MEM structure.
Still another advantage is that the MEM transmission formed using the five-level polysilicon process of the present invention can be used as a surety interlock to prevent unauthorized operation of a MEM device until enabled to bring a bridging gear set into position to complete an otherwise incomplete or interrupted gear train and thereby permit power to be transferred from a motive source (e.g. an electrostatic motor) to a load.
These and other advantages of the method of the present invention will become evident to those skilled in the art.